1. Field of the Invention
The present invention relates to a backlight unit and method for fabricating the same for a display. More particularly, the present invention relates to a backlight unit suitable for reducing humming noise that takes place during dimming adjustment of the backlight unit having external electrode lamps.
2. Discussion of the Related Art
The CRT (Cathode Ray Tube) has traditionally served as the basis for televisions, monitors, information terminals, etc. However, CRTs have disadvantages with regard to, for example, size and weight.
Other display technologies have emerged, which address the size and weight problems associated with CRTs. Among these are Liquid Crystal Displays (LCD), which utilize an electro-optical effect; Plasma Display Panels (PDP), which utilize a gas discharge; and Electro Luminescent Displays (ELD), which utilize an electro luminescent effect. In particular, research in LCD technology has been progressing rapidly.
In efforts to replace CRTs, LCDs have been developed to have minimal physical dimensions, low weight, and low power consumption. As a result, LCDs are now used in applications such as laptop computers and desktop monitors. As performance has improved, LCDs have also become popular in large-scale information display applications.
Most liquid crystal display devices are light receptive devices that display a picture by adjusting the quantity of light received from outside the liquid crystal display device. As such, LCDs require a separate light source for directing the light to the LCD panel. This separate light source is referred to as a backlight unit.
Related art backlight units include an arrangement of cylindrical lamps. The cylindrical lamps are arranged in either edge type or direct type configurations.
The edge type backlight unit has a lamp unit mounted on an edge of a light plate which guides the light. The lamp unit is provided with a lamp for emitting the light, a lamp holder placed on both ends thereof for protecting the lamp, and a lamp reflective plate surrounding an outside surface of the lamp having one side placed on the edge of the light plate for reflecting the light from the lamp toward the light plate.
The edge type backlight unit, which has the lamp unit mounted on the edge of the light plate, is mostly applicable to laptop or desktop computer monitors, for which minimal physical dimensions are important. The edge type backlight unit has good light output, uniformly long lifetime, and can be very thin.
Direct type backlight units, originally developed for LCD displays greater than 20 inches, have a plurality of lamps arranged in parallel under a diffusion plate for directing the light toward a front surface of the LCD panel. Direct type backlight units have a better light utilization efficiency than edge type backlight units. As such, direct type backlight units are mainly used in large sized LCDs that require high luminance.
However, because LCDs having direct type backlight units, such as large sized monitors, TVs, or the like, have a longer time period of use and a greater number of lamps as compared to laptop computers, direct type backlight units have a higher rate of lamp failure than edge type backlights.
Moreover, in the case of the edge type backlight unit that has the lamp units mounted on opposite edges of the light plate, if one lamp fails, there will be drop in screen luminance, but otherwise there will be no significant problems. However, because the direct type has the plurality of lamps under the screen, if one lamp fails, the portion of the screen in front of the failed lamp will become significantly darker compared to the rest of the screen. As such, the portion where the lamp fails to turn on appears on the screen as a nonuniformity in luminance.
Due to this, because frequent replacement of the lamps is required for the direct type LCD, the direct type LCD is required to have a structure in which disassembly and assembly of the backlight unit is easy.
FIG. 1 illustrates a perspective view of a related art direct type backlight unit 100. Referring to FIG. 1, the related art backlight unit 100 is provided with a plurality of lamps 11 arranged over a bottom cover 15 at regular intervals each with external electrodes 10a, and 10b at outside opposite ends of a tube, gripper type common electrodes 13 for fixedly securing the lamps 11 and applying a voltage to the electrodes 10a, and 10b at the opposite ends of the lamps 11, and supporter side bottoms 14a, and 14b arranged under the opposite ends of the lamps 11 in one direction for seating the common electrodes 13, respectively. Each of the common electrodes 13 is formed as a unit and arranged in one direction for applying the same voltage to the opposite ends of the plurality of lamps 11.
The lamps 11 each have external electrodes 10a, and 10b at opposite ends of a tube for receiving the voltage through the common electrodes 13.
The backlight unit 100 with the external electrode lamps has a burst mode inverter applied thereto to enable dimming control to improve contrast ratio and vary luminance. The dimming control is done by repetitively turning on/off of the lamps at about 150˜240 Hz frequency. However, the repetitive application of AC power to the common electrodes 13 at such a frequency causes vibration to occur not only at the lamps, but also at the common electrodes having the power applied to them.
In general, human beings can hear frequencies in the range of 20 Hz˜20000 Hz. Accordingly, changing the dimming control by applying a voltage to the common electrodes causes a vibration in this frequency range, which human beings can hear as a humming noise.